We are conducting research to examine the involvement of melatonin in the regulation of circannual reproductive rhythms, especially as it influences the hypothalamo-hypophyseal axis. Sheep are the animal model being used. In contrast to many other seasonally breeding species, sheep are short-day breeders; thus, exogenous melatonin should enhance, rather than inhibit reproductive performance. Experiments to examine the effect of melatonin on the ability of the anterior pituitary gland to release luteinizing hormone and on the ability of the hypothalamus to secrete gonadotropin-releasing hormone are in progress. In addition, we are examining the relationship between receptors for melatonin, estradiol-17 beta and gonadotropin-releasing hormone and the concentrations of gonadotropins in the anterior pituitary gland of ewes at different stages of the annual reproductive cycle.